Optical signals carried over fiber optic networks provide an optimal means of transmitting information. In basic principle, a laser signal source flashes light into one end of a fiber optic link. These flashes of light travel through the fiber optic cable. When the flashes of laser light reach the other end of the fiber optic link, they are detected by a light detector.
In this manner, information can be coded into the flashes of laser light emitted by the laser and decoded after the optical signal is detected by the light detector. Such an optical signal can carry large amounts of data and is, therefore, an excellent medium of transmitting video signals, for example, CATV or digital CATV signals.
One method of coding information into an optical signal is to vary or modulate the intensity of the flashes. Circuitry receiving the output from the light detector can then decode the transmitted data into a useable form. This is called amplitude modulation. Due to the non-linearity generated by the laser, it will produce distortion signals. These distortion signals will interfere with the original signal.
This problem with distortion in signal processing due to the non-linearity of a signal processing device is not limited to the example given here of a laser transmitting an optical signal. The difficulty with distortion caused by non-linearity arises in many signal processing contexts. For example, a circuit element as basic as an amplifier amplifying an electronic signal may be non-linear and distort the processed signal under some circumstances.
In the context of the present example, distributed feedback lasers (DFB lasers) are preferred for the transmission of optical data signals. The performance of DFB lasers is limited primarily by second-order distortion. Particularly, the optical modulation depth (OMD) that can be created by a laser being used as the source of an optical signal is limited by this second-order distortion. This is important because the OMD determines the quality of the signal transmitted.
For example, an optical signal may be used to transmit a video signal using an AM frequency carrier signal. In such a signal, the carrier to noise ratio, which defines the clarity of the signal, is proportional to the square of the OMD. Thus, using some means to reduce second-order distortion will help by increasing the optical modulation depth and greatly improve the dynamic range of the transmission. The result is a superior image at the end of the video transmission.
Accordingly, there is a need for a method and apparatus which can compensate for and correct distortion in non-linear signal processing elements, e.g. a non-linear laser light source generating a modulated optical signal.
Various attempts have been made to address this problem. In basic principle, the popular solution calls for a predistortion circuit which is used to distort the signal before it is processed by the non-linear processing element. This predistortion is matched to the distortion that will be caused during processing by the non-linear processing element so as to cancel out that distortion.
For example, U.S. Pat. No. 5,132,639 issued Jul. 21, 1992 to Blauvelt et al. proposed a predistortion circuit which is illustrated in FIG. 1. As shown in FIG. 1, an RF input signal on line 12 is fed into a directional coupler 10. The directional coupler 10 splits the input signal into a primary electrical path 13 and two secondary electrical paths 14 and 16. Typically the signal on the primary electrical path 13 is substantially higher in power than the signal on the secondary electrical paths 14 and 16. For example, an 11 dB directional coupler 10 may be used to achieve this result.
Typically, the primary electrical path 13 includes a time delay adjustment element 23 to produce a delayed modulation signal on line 24. The delayed modulation signal is then input to a second directional coupler 11.
The first secondary electrical path 14 includes, in series, distortion generator 15, an amplitude adjustment circuit 17, a frequency adjustment circuit 19, and a fine phase adjustment circuit 21. The signal output by the fine phase adjustment circuit 21 is provided on line 22 to the second directional coupler 11.
The signal output by the distortion generator 15 includes non-linear distortion of the input signal frequencies. The distortion in the output signal from the distortion generator is matched in magnitude with 180 degrees phase shift to the distortion inherently caused by the transmission device (not shown), e.g. a laser generating an optical signal, which receives and transmits the output signal on output line 25.
The second directional coupler 11 combines the signal from the primary path 24 with the distorted signal from line 22. The distortion produced by the transmission device will be cancelled out by the predistortion signal. So that, ideally, a signal with no remaining distortion is received after transmission.
The first secondary path 7 in FIG. 1 may be supplemented by a second secondary path 6. Like the first, the second secondary path 6 includes, in series, a distortion generator 35, an amplitude adjustment circuit 37, a frequency adjustment circuit 39 and a fine phase adjustment circuit 41. Second secondary path 6 provides an output signal on line 42 which is combined with the output signal on line 22 from the first secondary path 7 by the second directional coupler 11. In this example, the second secondary path 6 would be used to generate third-order distortion in the transmission signal to cancel third-order distortion caused by the transmission device (not shown).
Another prior attempt to solve the non-linear distortion problem is found in U.S. Pat. No. 5,119,392 issued Jun. 2, 1992 to Childs. Childs attempts to correct second-order distortion by using a field effect transistor (FET) biased for square law operation to make corrections in the transmission signal. The FET is connected in-line with a laser generating an optical transmission signal. Due to field and doping dependent variations in the carrier mobility within the transistor, it is difficult to obtain a FET with ideal square law operation. The performance of this type of predistortion circuit is limited.
Accordingly, there remains a need in the art for a very simple method of generating predistortion to counter the distortion created by a non-linear signal processing element, e.g. a optical signal generator.